This invention relates to a method for polishing a target surface for forming an oxide superconductor thin film on the surface of a tape-shaped substrate, as well as to a substrate for an oxide superconductor. In particular, this invention relates to a substrate for an oxide superconductor obtained by polishing the target surface of a tape-shaped metallic substrate with a level of smoothness on the order of nanometers and forming an intermediate layer thereon.
Oxide superconductors are superior superconductors having critical temperatures beyond the liquid nitrogen temperatures.
Typical examples of tape-shaped superconductor wire material include tape-shaped wire materials obtained by forming a polycrystalline oriented film of yttria-stabilized zirconia (YSZ) crystalline direction-controlled for example by the IBAD (Ion Beam Assisted Deposition) method on the surface of a Hastelloy® alloy tape comprising a Ni alloy and further forming an oxide superconductor film of YBCO type (such as YBa2Cu3O7-y) as disclosed, for example, in Japanese Patent Publication Tokkai 9-120719.
There are currently many kinds of problems to be solved in order to make the oxide superconductor materials of this kind practically usable.
One of these problems is that oxide superconductors have low critical current densities. A main cause of this is that there exists electrical anisotropy in the crystal structures of oxide superconductors. It is known in particular that oxide superconductors can easily allow an electric current to flow in the directions of the a-axis and the b-axis of their crystalline axes but a current cannot easily be made to flow in the direction of the c-axis. In order to form an oxide superconductor on a substrate and to use it as a superconductor, therefore, it is necessary to form on the substrate a superconductor with good crystalline directionality, to orient the a-axis or the b-axis of the crystal of the oxide superconductor in the direction intended for the flow of the electric current, and to orient the c-axis of the oxide superconductor in another direction.
As a method for achieving the above, Japanese Patent Publications Tokkai 6-145977 and 2003-36742, which are herein incorporated by reference, disclose a method of providing the surface of an elongated tape-shaped metallic substrate with an intermediate layer with controlled crystalline directionality and to form a film of oxide superconductor thereabove.
If the directionality of the intermediate layer is improved, the directionality of the superconductor film formed thereabove is also improved. For obtaining a superconductor film with high critical current (Ic) and critical current density (Jc), it is particularly essential to obtain a high level of biaxial directionality.
Since the directionality of the intermediate layer to be formed for this purpose depends upon the crystalline characteristic of the surface of the underlying tape-shaped substrate, the crystalline direction and the in-plane directionality of the tape-shaped substrate become important for obtaining an intermediate layer with good directionality.
For crystallizing the intermediate layer film such that good directionality is obtained, it is necessary to finish the surface of the tape-shaped substrate so as to have smoothness and flatness on the level of several nanometers.
U.S. Pat. No. 6,908,362 disclosed another method according to which the surface of a nickel or nickel alloy tape accurately and an oxide superconductor thin film thereafter.
Another problem that is preventing superconductivity from becoming practically usable relates to the behavior of magnetic flux quanta. The Lorentz force generated by the flow through a superconductor tends to move magnetic flux quanta but the motion of magnetic flux quanta generates heat and the superconducting condition becomes destroyed. Thus, it becomes possible to increase the critical current (Ic) that can be passed through a superconductor if magnetic flux quanta can be pinned so as not to move. Japanese Patent Publications Tokkai 6-145977 and 2003-36742 are prior art documents that teach the importance of polishing the surface of the base material so as to be smooth and flat.
It is technologically difficult, however, to polish the surface of a tape-shaped substrate to the level of several nanometers by a mechanical polishing process, and there has not been any example of success.